Filters can be a mixed blessing.
Filters can be very helpful but, consider that for middle schooler, filters can be circumvented and can even be seen as a challenge to try to outmatch parental control. If you choose to use filters, it is a good idea to let your child know you're implementing filtering.

The suggestions below are from CommonSenseMedia.com
http://www.commonsensemedia.org/internet-filters-tips#tip_answer_4
  • Don’t depend on filters alone.
    Middle school kids can defeat filters. Good old-fashioned parenting and Internet safety rules must be part of the picture. This means that you must be clear about what is and isn’t acceptable.
  • Consider checking browser histories.
    By middle school, your children will have outgrown filters. Many parents choose to check where their kids been going recently. You can do this by clicking on the arrow next to the address bar or by going into the menu and clicking on “history.” Your kids will consider this spying, so decide ahead of time how you’ll talk to them if you find that they’ve been visiting troubling sites.
  • Check in often.
    Periodically ask your kids to show you what they’ve found that they like. Showing that you’re interested in the good stuff they’ve been finding may mean they’re more comfortable coming to you if something on a site bothers them. Make sure kids know to check in if they see something that’s hateful, pornographic, or violent.
  • Monitoring software has plusses and minuses.
    If you don’t think your kids are using good judgment, monitoring software will help you figure out if your suspicions are warranted. Parents struggle with whether or not to use these tools because they want to respect their teens’ privacy. Think about how your teens will react if they find out.

Free Filtering Software link http://www1.k9webprotection.com/ A National Parenting Center and iParenting Media awards winner.

A review of good commercial filters is available at http://internet-filter-review.toptenreviews.com/
The top ones were:
Net Nanny Parental Controls
PureSight PC
CYBERsitter
Safe Eyes
CyberPatrol
McAfee Family Protection
FilterPak
imView